1925 Eileen Dam Failure - Know Your Foundation!

The Eileen Dam was constructed in 1923 through mid-August 1924 on the Moyie River, located in northern Idaho. The dam was originally built to provide power to the mining operations in the area. The dam was designed as a concrete arch dam with a timber lined spillway around the left (east) side and included a powerhouse with one turbine. The arch dam was approximately 150 feet long, 58 feet tall, 2 feet thick at the top to 12 feet thick at the base and had a radius of 65 feet. The estimated construction cost in 1924 ranged between $75k to $100k (present value between $1.4M to 1.9M dollars). 

On Tuesday, May 19, 1925, nine months after construction was completed and before the turbine had produced any electricity, while the forebay was full due to runoff from spring thaw, the dam failed. The initial failure point was the contact where the left (eastern) abutment and spillway were bearing on the bedrock, and the foundation materials were washed away which resulted in a breach of the dam. Various sources reported speculation that the failure was due to undermining of the spillway on the left side, excessive water pressure loading on the face of the dam exceeding the capacity of the foundation, or a rupture due to increase in foundation hydrostatic pressure from seepage through the joints in the rock. 

Witnesses stated that the rock layers in the abutment “peeled off like shingles on a roof during a windstorm” as the river forged a new channel around the dam. The failure caused a 10-foot rise in the river downstream of the dam, and the rushing waters damaged a rock retaining wall and iron railing below the Moyie Falls power plant (which was not in operation at the time) about 4 miles downstream. The Eileen Dam failure resulted in no fatalities. 

This dam failure is an important reminder of how critical a thorough understanding of the foundation conditions is for structures. The mechanism of failure teaches us that simply founding a dam on “bedrock” does not necessarily guarantee sufficient performance since not all rock is created equal. 

The majority of the concrete arch portion of the dam survived the breach and still stands today. This existing structure cuts more than halfway across the river, constricting the river and creating rapids where the left abutment used to be, and has become a popular spot for whitewater rafters and hikers. 

This paper and presentation will present an overview of the failure, discuss postulated failure mechanisms, and highlight the importance of “knowing your foundation”!